You know that if you add a different header image (that is full size) you don't need to go to photoshop and put it on top the the original image, right? You can just use the new image on it's own. Of course if I'm misunderstanding and you're not doubling the images then ignore what I just said.
As for your question about uploading and resizing an image...I don't think there is what you're looking for. Possibly could be something if you got into js or flash maybe but I don't know.
And I do love doing this...I just wish I was able to devote more time to it.